Device for measuring rotor imbalance

Abstract

The invention relates to a device for measuring a rotor imbalance. Said device comprises a main shaft which is rotatably mounted in a machine housing and to which the rotor to be measured is fixed, a single-phase alternating-current motor for driving the main shaft, with a single-phase winding ( 4 ) and an auxiliary winding ( 5 ) which is offset in phase for driving the main shaft ( 1 ) up to measurement speed and a control device ( 17 ) that adjusts the current of the motor. Said control device ( 17 ) has an assembly ( 8, 6; 7; 20; 22, 23 ) which reduces the supply current delivered to the two windings ( 4, 5 ) of the motor ( 3; 21 ) once the measurement speed has been reached, in such a way that the motor creates a reduced torque which substantially compensates a speed drop produced by friction and drag.

What is claimed is:  
     
       1. Apparatus for measuring a rotary member unbalance comprising a main shaft which is mounted rotatably in a machine housing and to which the rotary member to be measured can be fixed, a single-phase ac motor operating from a supply current for driving the main shaft, the motor having a single-phase main winding, an auxiliary winding which is displaced in phase relationship from the main winding for driving the main shaft up to a measuring speed, a control device which adjusts the supply current to the windings, the control device includes a supply current-reducing device which when the measuring speed is reached, reduces the supply current supplied to both windings of the motor so that the motor produces a reduced torque which substantially compensates for a drop in speed caused by friction and air resistance. 
     
     
       2. Apparatus as set forth in claims  1  wherein the device for reducing the supply current is operable to 
       adjust the supply current when the measuring speed is reached for a rise in speed up to an upper speed limit to a first value for producing a first reduced torque,  
       adjust the supply current the upper speed limit is reached for a drop in speed to a lower speed limit to a second value for producing a second reduced torque, and  
       again adjust the supply current to the first value when the lower speed limit is reached.  
     
     
       3. Apparatus as set forth in  claim 1  wherein the supply current-reducing device is controlled by a two-step controller which is set in dependence on the rotary member speed. 
     
     
       4. Apparatus as set forth in  claim 1  wherein the main and the auxiliary windings are substantially identical windings. 
     
     
       5. Apparatus as set forth in  claim 1  wherein the device for reducing the supply current is an ohmic resistor or reactance. 
     
     
       6. Apparatus as set forth in  claim 5  wherein the reactance is an inductance. 
     
     
       7. Apparatus as set forth in  claim 1  wherein the device for reducing the supply current is in the form of a transformer. 
     
     
       8. Apparatus as set forth in  claim 1  wherein the device for reducing the supply current is in the form of an ac voltage converter. 
     
     
       9. Apparatus as set forth in  claim 8  wherein the ac voltage converter is a semiconductor ac voltage converter which is controlled by pulse width modulation in dependence on the speed of the rotary member. 
     
     
       10. Apparatus as set forth in  claim 1  further comprising additional windings, wherein when the measuring speed is reached, the additional windings are added to the main and auxiliary windings to reduce the torque produced by the motor. 
     
     
       11. Apparatus as set forth in  claim 6  wherein the reactance is a choke.